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<h1>Hercules Version 4: TCP/IP networking with Hercules</h1>
<p>
This page describes how to set up TCP/IP connectivity between
a Hercules machine and the outside world.
<p>
Since Hercules runs as a user process under the control of a driving
system (usually Linux/x86 or Windows), it does not have direct
access to the driving system's network adapter.  This presents a
problem in establishing connectivity between the network and the
TCP/IP stack of an operating system running under Hercules.
<p>
But thanks to a technique originally demonstrated by
Willem Konynenberg, it is possible to establish a virtual
point-to-point link between the TCP/IP stack running under
Hercules and the TCP/IP stack of the driving system.  The
driving system is then used as a router to pass IP frames
between the Hercules TCP/IP stack and the rest of the network,
as shown in the below diagram.
<p>
<code><pre>
        +--------------------------------+
        |     Linux/x86 Driving System   |
        |                                |
        +-------------+                  |
        |  Hercules   |                  +--------+
        |-------------|                  |  eth0  |
        |   OS/390    |      TCP/IP ------------------> Network
        |   TCP/IP    |                  |10.1.2.1|
        |-------------|         |        +--------+
        |    CTCA     |         |        |
        |192.168.200.1|         |        |
        +------|------+   192.168.200.2  |
        |  /dev/tun           tun0       |
        |      |                |        |
        |      +----------------+        |
        |       Virtual CTC link         |
        |                                |
        +--------------------------------+
</pre></code>
<p>
The virtual CTC link is provided by the Universal TUN/TAP driver
developed by Maxim Krasnyansky.  This driver creates a tunnel
which appears to Hercules as a character device (/dev/tun0 or
/dev/net/tun) and appears to the driving system as a virtual
network interface (tun0).
The Hercules 3088 driver makes the tun device appear as
a CTCA (Channel to Channel Adapter) to the S/390 operating system
running under Hercules.  Each end of the link has its own IP
address which is distinct from the IP address of the driving
system's real network adapter.
<p class="note">
    <i><b>Windows users</b> &nbsp; should refer to Fish's
    <a href="http://www.softdevlabs.com/ctci-win">CTCI-WIN</a>
    web page which provides similar "TunTap" functionality
    on Windows platforms.</i>

<h2>Installing the TUN/TAP Driver (Linux 2.4)</h2>
<p>
The TUN/TAP driver is delivered as part of the Linux 2.4 kernel, and
if you are using one of the popular Linux distributions you will find
that the TUN/TAP driver is already installed.  If not, then you must
rebuild the kernel with the configuration option CONFIG_TUN=m specified.
<p>
Note that the version of TUN/TAP in Linux 2.4 differs from the earlier
version in that it allows access to all TUN interfaces (tun0, tun1, etc)
through a single character device /dev/net/tun, instead of defining
multiple devices /dev/tun0, /dev/tun1, etc.
<p>
The procedure for completing the TUN/TAP setup for Linux 2.4 is shown below.
<ol>
<li>Use these commands to create the TUN device:
    <code>
    <br>su </code>(enter the root password when prompted)<code>
    <br>mkdir /dev/net
    <br>mknod /dev/net/tun c 10 200
    <br>chgrp <em>xxxxx</em> /dev/net/tun
        </code>(where <em>xxxxx</em> is the group under which
                you run Hercules)<code>
    <br>chmod g+rw /dev/net/tun
    <br>chmod o-rw /dev/net/tun
    </code>
<li>Edit the file <b><i>/etc/modules.conf</i></b> (it is called
    <b><i>/etc/conf.modules</i></b> in some distributions)
    and add the following line:
    <br><code>alias char-major-10-200 tun</code>
    <br>This causes the TUN/TAP driver to be loaded automatically
    when the /dev/net/tun device is opened by Hercules.
</ol>

<h2>Installing the TUN/TAP Driver (Linux 2.6, FreeBSD, OS X)</h2>
<p>
For distributions based on the Linux 2.6 kernel you will probably find that the
TUN/TAP driver is already installed and the /dev/net/tun device is already
defined. If not, then follow the procedure for Linux 2.4 as descibed above.
<p>
For FreeBSD, refer to <code>man tun</code>.
You will likely want to enable
<code>net.link.tun.devfs_cloning</code> to create
<code>/dev/tun</code>, but you will be in uncharted waters.
<p>
If you are using a current distribution
you will need to alter the permissions on the /dev/net/tun device
to allow Hercules to open it.
Issue <code>ls -l /dev/net/tun</code> (Linux)
<code>ls -l /dev/tun0</code> (OSX) to determine whether Hercules can
open the device.
<pre>
[root@servus ~]# ls -l /dev/net/tun
crw-rw---- 1 root staff 10, 200 Nov 24 15:01 /dev/net/tun

[enrico@enrico-mbp ref.obj]$ls -l /dev/tap0
crw-rw-rw-  1 root  wheel   21,   0 Nov 22 18:31 /dev/tap0

[enrico@enrico-mbp ref.obj]$ls -l /dev/tun0
crw-rw-rw-  1 root  wheel   19,   0 Nov 22 18:31 /dev/tun0
</pre>
<p>You have two ways to make the <code>tun</code> device usable.
Use the second only with nonstandard and/or old kernels
(that is, if the first udev rule does not work for you.)
<ol>
<li>
The preferred way to change properties for the Linux <code>tun</code>
device is via a udev rule in
<code>/etc/udev/rules.d</code>
<pre>
[root@servus rules.d]# cat 99-jph.rules
KERNEL=="tun",                  GROUP="staff", MODE="0660"
</pre>
<li>Alternatively, use the following commands to set the necessary permissions:
    <code>
    <p>su </code>(enter the root password when prompted)<code>
    <br>chgrp <em>xxxxx</em> /dev/net/tun
        </code>(where <em>xxxxx</em> is the group under which
                you run Hercules)<code>
    <br>chmod 0660 /dev/net/tun
    </code>
    <p>or make the device file writable by world; this does not consist an
    integrity exposure, as the world at large cannot do anything with the
    file descriptor it gets.
    <code>
    <p>chmod 0666 /dev/net/tun
    </code>
<p><i>Additional notes from Greg Smith:</i>
    <ol>
    <li>I find on my Fedora Core 6 system that I have to add the above 2 commands
        to <b><i>/etc/rc.local</i></b>
        and update <b><i>/etc/udev/rules.d/50-udev.rules</i></b> replacing
        <br><code>KERNEL=="tun", NAME="net/%k"</code>
        <br>by
        <br><code>KERNEL=="tun", NAME="net/%k", GROUP="<em>xxxxx</em>", MODE="0660"</code>
    <li>In the hercules log you should see /dev/net/tun0 opened. I get a couple
        of error messages about SIOCDIFADDR and SIOCSIFHWADDR ioctl's failing
        but these can be ignored.
    </ol>
</ol>
<p>
In Linux 2.6 the file <b><i>/etc/modules.conf</i></b> no longer exists,
instead there is a file called <b><i>/etc/modprobe.conf</i></b>.
TUN/TAP will usually work, however, without any change to the modprobe
configuration.

<h2><a name="hercifc">Configuring a TUN interface</a></h2>
<p>You may be able to configure the tunnel interfaces you will need
at system boot;
but if you cannot or will not, Hercules supplies a utility
(<code>hercifc</code>) to do this.
<h3>At Linux boot</h3>
<p><code>openvpn</code> (<a href="http://openvpn.net/">http://openvpn.net/</a>)
is a utility that can open a tunnel device permanently.
In addition you are likely to need a udev rule as described above.
Openvpn is usually installed as a package (<code>yum install openvpn</code> on Fedora);
if not refer to <a href="http://openvpn.net/index.php/open-source/documentation/howto.html#install">
http://openvpn.net/index.php/open-source/documentation/howto.html</a>.
Note, however, that <code>openvpn</code> is just one way to create a permanent tunnel device;
alternative methods may be possible.
<p>
To create permanent a tunnel interface, add this to rc.local
(or as appropriate for
the distribution you are using):
<pre>
    openvpn --mktun --dev tun0 --group hercules
    ifconfig tun0 192.168.2.65 pointopoint 10.0.0.33
    echo 1 >/proc/sys/net/ipv4/ip_forward
</pre>
<p>This example assumes that Hercules is in the group
<code>hercules</code>; it shows the tunnel device in a separate network,
which requires routing as described below.
<code>ifconfig</code> can display the permanent interface:
<pre>
[root@servus ~]# ifconfig tun0
tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:192.168.2.65  P-t-P:10.0.0.33  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:71282 errors:0 dropped:0 overruns:0 frame:0
          TX packets:50 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:4289708 (4.0 MiB)  TX bytes:2764 (2.6 KiB)
</pre>
<p>
If the interface is in the same subnet as the Hercules host,
you need to enable proxy arp.
One way to achieve this is to turn it on dynamically:
<pre>
    echo 1 >/proc/sys/net/ipv4/conf/eth0/proxy_arp
</pre>
<h3>Free BSD</h3>
<p>
On FreeBSD, use the command <code>ifconfig tun<n> create</code> to create
an interface.
<h3>Using hercifc</h3>
<p>
The tun0 network interface in the driving system must be configured
as a point-to-point link.  The original design of the TUN/TAP driver did not
allow the interface to be statically configured like a regular network
interface &#151; the tun0 interface does not exist until a program
opens the TUN device.  For this reason, Hercules provides a special
program called hercifc to configure the tun0 network interface.  This
program is launched automatically by Hercules 3088 CTC device
initialization.
(Later, the ability to make an interface permanent was added;
this is why <code>openvpn</code> and preconfigured interfaces work.)
<p>
To allow the hercifc program to issue the necessary configuration
commands, you must ensure that hercifc is installed with setuid root
file permissions.  When Hercules is built with the
configuration option --enable-setuid-hercifc=xxxxx, <b><i>make install</i></b>
will install hercifc in /usr/local/bin with setuid root permissions.
The hercifc program will be executable only by group xxxxx.
<i>Note: Unrestricted access to the hercifc program could present a
potential security exposure, so you will want to ensure that hercifc
can be executed only by the group which is authorized to run Hercules.</i>
<p>
The following commands have the same effect as the
configuration option --enable-setuid-hercifc=xxxxx:
    <code>
    <p>su </code>(enter the root password when prompted)<code>
    <br>chgrp <em>xxxxx</em> /usr/local/bin/hercifc
        </code>(where <em>xxxxx</em> is the group under which
                you run Hercules)<code>
    <br>chmod 4750 /usr/local/bin/hercifc
    <br>exit
    </code>

<p>
<em>Note:</em> you can avoid the need to install hercifc as a setuid root
program by creating a permanent tunnel interface at Linux boot time,
as described above.

<h2>Enabling IP forwarding</h2>
<p>
You must ensure that your kernel is enabled for IP forwarding.
Popular Linux distributions usually have a configuration option
to enable IP forwarding or routing:
<ul>
<li>For RedHat, specify <code>net.ipv4.ip_forward=1</code> in
    the <i><b>/etc/sysctl.conf</b></i> file.
<li>For SuSE, specify <code>IP_FORWARD="yes"</code> in
    the <i><b>/etc/rc.config</b></i> file.
<li>If you cannot find this option in your distribution, the following
    command should work on any Linux 2.x and later kernel:
    <br><code>echo "1" > /proc/sys/net/ipv4/ip_forward</code>
</ul>

<h2>Defining a route to Hercules TCP/IP</h2>
<p>
Client systems that connect to TCP/IP applications running in
the Hercules machine need to have a routing entry to define
the driving system as the gateway into the Hercules system.  An
example route definition for a Unix client system is shown below:
<code><pre>
route add 192.168.200.1 gw 10.1.2.1
</pre></code>
<p>
For a Windows client, go to Settings -> Control Panel -> Network ->
Configuration -> TCP/IP -> Properties -> Gateway
and add the driving system's IP address to the list of gateways.
Alternatively, enter a route command such as:
<code><pre>
route add 192.168.200.0 mask 255.255.255.0 10.1.2.1 metric 1
</pre></code>
<p>
If you want to avoid having to update client systems, another way
is to add an appropriate routing entry to your default gateway router.

<h2>Defining the link in Hercules</h2>
<p>
You must define a CTC device pair in the Hercules configuration
file:
The first device number must be even.
The second device must bear the same definition as the first
instance and be at device number + 1
(for CTCI, the first definition beyond the device type is ignored;
you only need to specify the definition with the second device).
Devices should preferably be grouped (furthermore, it makes the
configuration file easier to read).
<p>
For a preconfigured tunnel:
<p>
<code>
<br>0E20.2 CTCI tun0
<br>or
<br>0E20,0E21 CTCI tun0
<br>or
<br>0E20-0E21 CTCI tun0
</code>
<p>
<p>Using hercifc:
<p>
<code>
<br>0E20.2 CTCI 192.168.200.1 192.168.200.2
<br>or
<br>0E20,0E21 CTCI 192.168.200.1 192.168.200.2
<br>or
<br>0E20-0E21 CTCI 192.168.200.1 192.168.200.2
</code>
<p>
<p>Check <A HREF="hercconf.html#device_definitions">Device Definition Statement syntax </A> for an explanation of device grouping.
<p>
Two IP addresses must be assigned, one for the driving system's
end of the link, and one for the Hercules end of the link.  For
this example I have chosen 192.168.200.1 for the Hercules IP
address, and 192.168.200.2 for the driving system's IP address.
Since this is a point-to-point link, any addresses may be chosen,
provided that the network part of the address (192.168.200 in
this example) does not conflict with any existing network
addresses used in your IP network.

<h2>Configuring the Hercules TCP/IP stack</h2>
<h4>TCP/IP for VSE</h4>
<p>
This is an example of the configuration statements which you need
to include in the IPINIT00.L member of PRD1.BASE:
<pre>
SET IPADDR   = 192.168.200.001
SET MASK     = 255.255.255.000
DEFINE LINK,ID=CTCE20,TYPE=CTCA,DEV=(E20,E21),MTU=1500
DEFINE ROUTE,ID=LINUX,LINKID=CTCE20,IPADDR=0.0.0.0
</pre>
<p>
The CTC devices should be defined to VSE using the following
statements in the $IPLxxx.PROC procedure in IJSYSRS.SYSLIB:
<pre>
ADD E20:E21,CTCA,EML
</pre>

<h4>TCP/IP for OS/390 or VM/ESA</h4>
<p>
This is an example of the configuration statements which you need
to include in the TCPIP.PROFILE.TCPIP dataset (OS/390), or in the
PROFILE TCPIP file on TCPMAINT 198 (VM):
<pre>
DEVICE CTCDEV1 CTC E20
LINK CTCLINK1 CTC 0 CTCDEV1
HOME 192.168.200.1 CTCLINK1
GATEWAY
; Network      First Hop     Link Name Size   Subnet Mask  Subnet Value
192.168.200.2  =             CTCLINK1  1500   HOST
DEFAULTNET     192.168.200.2 CTCLINK1  1500   0
START CTCDEV1
</pre>
<p>
For OS/390, the CTC devices need to be defined as device type 3088 in
the IODF.  Use the <code>D U,CTC</code> command to find out which 3088
addresses are defined in your IODF.
<p>
For VM, the CTC devices must be attached to the TCPIP virtual machine.
<p>
Because TCP/IP uses long running channel programs, the missing interrupt
handler should be disabled for the CTC devices.
For OS/390, add this statement in PARMLIB member IECIOS00:
<pre>
MIH TIME=00:00,DEV=(E20-E21)
</pre>
<p>
For VM, add this command to the PROFILE EXEC file of OPERATOR 191:
<pre>
'CP SET MITIME 0E20-0E21 OFF'
</pre>

<h4>Linux for S/390</h4>
<p>
This is an example of the network definitions which you need
in a Linux/390 system running under Hercules:
<pre>
ifconfig ctc0 192.168.200.1 pointopoint 192.168.200.2 mtu 1500
route add defaultroute gw 192.168.200.2
</pre>
<p>
Linux/390 will autodetect the CTC devices E20 and E21
at startup and will assign the interface name ctc0.

<h2>What to do if TUN/TAP doesn't work</h2>
<p>
Check the following (thanks to Richard Higson for this checklist):
<ol>
<li>Enter the command <code>ls -l /dev/tun0 /dev/net/tun</code>.
    <br>For Linux 2.4 and later kernels, the response should be:
    <code>
      <br>ls: /dev/tun0: No such file or directory
      <br>crw-rw---- 1 root <i>xxxxx</i> 10, 200 Sep 13 07:06 /dev/net/tun
    </code>
    <br>For Linux 2.2, the response should be:
    <code>
      <br>crw-rw---- 1 root <i>xxxxx</i> 90, 0 Feb 3 2001 /dev/tun0
      <br>ls: /dev/net/tun: No such file or directory
    </code>
    <br>(<i>xxxxx</i> should be the group under which you run Hercules).
<li><code>ls -l /usr/local/bin/hercifc</code> should show
    <code>
      <br> -rwsr-x--- 1 root <i>xxxxx</i> 17333 Dec 31 20:55 /usr/local/bin/hercifc
    </code>
    <br>(<i>xxxxx</i> should be the group under which you run Hercules).
<li>When hercules comes up, and <i>before</i> IPLing your favorite OS, verify
    that you have your underlying network stuff up and ready to roar:
    <code><pre>
[root]# <b>ifconfig</b>
eth0      Link encap:Ethernet  HWaddr 00:12:34:56:78:9A
          inet addr:10.1.2.1  Bcast:10.255.255.255  Mask:255.0.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
tun0      Link encap:Point-to-Point Protocol
          inet addr:192.168.200.2  P-t-P:192.168.200.1  Mask:255.255.255.0
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
[root]# <b>netstat -in</b>
Kernel Interface table
Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR   TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0   1500   0     201      0      0      0     196      0      0      0 BMRU
tun0   1500   0       0      0      0      0       0      0      0      0 MOPRU
[root]# <b>netstat -rn</b>
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
10.0.0.0        0.0.0.0         255.0.0.0       U        40 0          0 eth0
192.168.200.0   0.0.0.0         255.255.255.0   U        40 0          0 tun0
127.0.0.0       0.0.0.0         255.0.0.0       U        40 0          0 lo
0.0.0.0         10.1.0.1        0.0.0.0         UG       40 0          0 eth0</pre></code>
<li><code>`cat /proc/sys/net/ipv4/ip_forward`</code> should show "1".
    If it doesn't, your L386 won't forward (==route) packets at all.
<li>Is the TUN/TAP driver loaded?
    <ol>
    <li>TUN/TAP compiled into the kernel (<code>`make menuconfig`</code>)
        look for <code>"CONFIG_TUN=m"</code> in <i><b>/usr/src/linux</i></b>
    <li><code>`lsmod`</code> after starting hercules should show
        <code>tun 3456 2 (autoclean)</code>
    </ol>
<li>Look for
    <code>
      <br>Dec 14 16:47:19 wie kernel: Universal TUN/TAP device driver 1.3 (C)1999-2000
      <br>Maxim Krasnyansky
    </code>
    <br>in syslog after starting hercules
</ol>

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